U.S. patent application number 10/420501 was filed with the patent office on 2004-02-26 for polarization-stabilized all-optical switch.
Invention is credited to Hamilton, Scott A., Ippen, Erich P., Robinson, Bryan S., Savage, Shelby J..
Application Number | 20040037489 10/420501 |
Document ID | / |
Family ID | 31891168 |
Filed Date | 2004-02-26 |
United States Patent
Application |
20040037489 |
Kind Code |
A1 |
Robinson, Bryan S. ; et
al. |
February 26, 2004 |
Polarization-stabilized all-optical switch
Abstract
Described is an all-optical switch that is significantly
insensitive to polarization instabilities. The optical switch can
be configured as an ultrafast logic gate, a switch for ultrafast
communication systems or a key component of an all-optical
regenerator. Performance is independent of the statistical
characteristics of the data controlling the switch. The switch
includes a birefringent optical channel in communication with one
end of a nonlinear optical channel through a coupler and a
polarization rotation mirror in communication with the other end of
the nonlinear channel. An optical data pulse for controlling the
switching function is provided to one port of the coupler.
Inventors: |
Robinson, Bryan S.;
(Belmont, MA) ; Savage, Shelby J.; (Cambridge,
MA) ; Hamilton, Scott A.; (Newton, MA) ;
Ippen, Erich P.; (Belmont, MA) |
Correspondence
Address: |
GUERIN & RODRIGUEZ, LLP
5 MOUNT ROYAL AVENUE
MOUNT ROYAL OFFICE PARK
MARLBOROUGH
MA
01752
US
|
Family ID: |
31891168 |
Appl. No.: |
10/420501 |
Filed: |
April 22, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60378746 |
May 8, 2002 |
|
|
|
Current U.S.
Class: |
385/16 |
Current CPC
Class: |
G02F 1/3515 20130101;
H04Q 11/0005 20130101; H04J 14/08 20130101; H04Q 2011/0035
20130101; G02F 1/3517 20130101; H04J 14/06 20130101; H04Q 2011/0041
20130101 |
Class at
Publication: |
385/16 |
International
Class: |
G02B 006/35 |
Goverment Interests
[0002] This invention was made with United States government
support under Contract No. F19628-00-C-002 awarded by the Defense
Advanced Research Project Agency (DARPA). The government may have
certain rights in the invention.
Claims
What is claimed is:
1. An optical switch comprising: a birefringent optical channel
having a first port and a second port, the birefringent optical
channel providing a pair of orthogonally polarized optical pulses
at the second port in response to a first optical pulse received at
the first port, the orthogonally polarized optical pulses being
separated by a delay time; a first coupler having a first port in
optical communication with the second port of the birefringent
optical channel, a second port adapted to receive a second optical
pulse, and a third port; a nonlinear optical channel having a first
port in optical communication with the third port of the first
coupler, and having a second port; and a polarization rotation
mirror in optical communication with the second port of the
nonlinear channel.
2. The optical switch of claim 1 wherein the polarization rotation
mirror is a Faraday mirror.
3. The optical switch of claim 1 wherein the polarization rotation
mirror provides a reflected optical pulse in response to an
incident optical pulse, the reflected optical pulse having a
polarization that is rotated to be orthogonal with respect to the
incident optical pulse.
4. The optical switch of claim 1 further comprising a control
optical channel in optical communication with the second port of
the first coupler.
5. The optical switch of claim 1 wherein the nonlinear optical
channel comprises an optical fiber.
6. The optical switch of claim 1 wherein the nonlinear optical
channel comprises a dispersion shifted optical fiber.
7. The optical switch of claim 1 wherein the nonlinear optical
channel comprises an optical semiconductor.
8. The optical switch of claim 1 wherein the first coupler
comprises a wavelength division multiplexer.
9. The optical switch of claim 1 wherein the birefringent optical
channel comprises a polarization maintaining optical fiber.
10. The optical switch of claim 1 wherein the birefringent optical
channel comprises a birefringent crystal.
11. The optical switch of claim 1 further comprising a circulator
having a first port adapted to receive the first optical pulse, a
second port in optical communication with the first port of the
birefringent optical channel and a third port to provide a first
output optical pulse.
12. The optical switch of claim 11 further comprising a bandpass
filter in optical communication with the third port of the
circulator.
13. The optical switch of claim 11 further comprising a bandpass
filter in optical communication with the second port of the
birefringent optical channel and the first port of the first
coupler.
14. The optical switch of claim 1 further comprising a bandpass
filter in optical communication with the second port of the
birefringent optical channel and the first port of the first
coupler.
15. The optical switch of claim 1 further comprising a polarization
dependent beamsplitter having a first port adapted to receive the
first optical pulse, a second port in optical communication with
the first port of the birefringent optical channel and a third port
to provide a second output optical pulse.
16. The optical switch of claim 15 wherein the polarization
dependent beamsplitter has a transmission polarization axis and the
birefringent optical channel has a fast axis and a slow axis, the
transmission polarization axis being oriented at approximately
45.degree. to the fast axis and the slow axis of the birefringent
optical channel.
17. The optical switch of claim 11 further comprising: an optical
source having a control terminal and having a source output port in
optical communication with the first port of the circulator, the
optical source configured to generate the first optical pulse at
the source output port in response to a control signal received at
the control terminal; and a clock recovery circuit having an input
port in optical communication with the third port of the circulator
and an output terminal in communication with the control terminal
of the optical source.
18. The optical switch of claim 17 further comprising a time delay
module in electrical communication with the clock recovery circuit
and in optical communication with the second port of the first
coupler, the time delay module delaying the second optical pulse in
response to a delay signal generated by the clock recovery
circuit.
19. The optical switch of claim 17 further comprising a time delay
module in electrical communication with the clock recovery circuit
and in optical communication with the first port of the circulator,
the time delay module delaying the first optical pulse in response
to a delay signal generated by the clock recovery circuit.
20. The optical switch of claim 17 wherein the clock recovery
circuit comprises a phase lock loop circuit.
21. The optical switch of claim 1 further comprising a second
coupler having a first port adapted to receive the second optical
pulse, a second port adapted to receive a third optical pulse, and
a third port in optical communication with the second port of the
first coupler.
22. The optical switch of claim 21 further comprising an optical
delay module in optical communication with the second port of the
second coupler, the optical delay module delaying the third optical
pulse relative to the second optical pulse by substantially the
delay time.
23. An optical switch comprising: a birefringent optical channel
having a first port and a second port, the birefringent optical
channel providing a first pair of orthogonally polarized optical
pulses and a second pair of orthogonally polarized pulses at the
second port in response to a first optical pulse and a second
optical pulse, respectively, received at the first port, the first
optical pulse and the second optical pulse being orthogonally
polarized, each of the pulses in the first pair and the second pair
of orthogonally polarized optical pulses being separated by a delay
time; a coupler having a first port in optical communication with
the second port of the birefringent optical channel, a second port
adapted to receive a third optical pulse, and a third port; a
nonlinear optical channel having a first port in optical
communication with the third port of the coupler, and having a
second port; a polarization rotation mirror in optical
communication with the second port of the nonlinear channel; a
polarization dependent beamsplitter having a first port, a second
port in optical communication with the first port of the
birefringent optical channel and a third port; a first circulator
having a first port adapted to receive the first optical pulse, a
second port in optical communication with the first port of the
polarization dependent beamsplitter and a third port adapted to
provide a first optical output signal; and a second circulator
having a first port adapted to receive the second optical pulse, a
second port in optical communication with the third port of the
polarization dependent beamsplitter and a third port adapted to
provide a second optical output signal.
24. A method for regenerating an optical data pulse, the method
comprising: transmitting the optical data pulse and a pair of
orthogonally polarized optical pulses through a nonlinear optical
channel in a forward direction, the optical data pulse and each of
the orthogonally polarized optical pulses having a forward
polarization orientation with respect to the nonlinear optical
channel, the orthogonally polarized optical pulses being separated
in time, the optical data pulse and one of the orthogonally
polarized optical pulses being substantially temporally coincident;
transmitting the optical data pulse and the orthogonally polarized
optical pulses through the nonlinear optical channel in a reverse
direction, the optical data pulse and each of the orthogonally
polarized optical pulses having a reverse polarization orientation
with respect to the nonlinear optical channel, each of the reverse
polarization orientations being orthogonal to the respective
forward polarization orientation, wherein an optical phase delay is
imparted to one of the orthogonally polarized optical pulses
relative to the other of the orthogonally polarized optical pulses
in response to the coincident transmission of the optical data
pulse and the one of the orthogonally polarized optical pulses
through the nonlinear optical channel; and delaying one of the
orthogonally polarized optical pulses relative to the other of the
orthogonally polarized optical pulses to generate a temporal
coincidence therebetween.
25. The method of claim 24 further comprising providing a
regenerated optical pulse in response to the optical phase delay
imparted to one of the orthogonally polarized optical pulses
relative to the other of the orthogonally polarized optical
pulses.
26. The method of claim 24 wherein the optical phase delay imparted
to one of the orthogonally polarized optical pulses relative to the
other of the orthogonally polarized optical pulses is an odd
integer multiple of 180.degree..
27. The method of claim 24 wherein the optical data pulse has a
wavelength that is not equal to a wavelength of the orthogonally
polarized optical pulses.
28. A method for performing a logical operation on a first optical
data bit and a second optical data bit, the first and second
optical data bits being separated by a predetermined time, the
method comprising: transmitting the first and second optical data
bits, a first optical clock bit and a second optical clock bit
through a nonlinear optical channel in a forward direction, the
first and second optical data bits and the first and second optical
clock bits each having a forward polarization with respect to the
nonlinear optical channel, the first and second optical clock bits
being orthogonally polarized and separated by a first time, the
first optical data bit being substantially temporally coincident
with the first optical clock bit and the second optical data bit
being substantially temporally coincident with the second optical
data bit; transmitting the first and second optical data bits and
the first and second optical clock bits through the nonlinear
optical channel in a reverse direction, the first and second
optical data bits and the first and second optical clock bits each
having a reverse polarization orientation with respect to the
nonlinear optical channel, each of the reverse polarization
orientations being orthogonal to the respective forward
polarization orientation, wherein an optical phase delay is
imparted to the first optical clock bit if the first optical data
bit is in an asserted state and wherein an optical phase delay is
imparted to the second optical clock bit if the second optical data
bit is in an asserted state; and delaying the first optical clock
bit relative to the second optical clock bit to generate a temporal
coincidence therebetween.
29. The method of claim 28 further comprising providing a result of
the logical operation in response to the optical phase delays
imparted to the first optical clock bit and the second optical
clock bit.
30. The method of claim 28 wherein the optical phase delay imparted
to one of the first optical clock bit and the second optical clock
bit is an odd integer multiple of 180.degree..
31. The method of claim 28 wherein the logical operation is an XOR
operation.
32. The method of claim 28 wherein the logical operation is a
complementary XOR operation.
33. A method for performing a logical operation on a first optical
data bit and a second optical data bit, the method comprising:
generating a pair of orthogonally polarized optical bits in
response to the first optical data bit, the orthogonally polarized
optical bits being separated in time, one of the orthogonally
polarized optical bits being substantially temporally coincident
with the second optical data bit; transmitting the orthogonally
polarized optical bits and the second optical data bit through a
nonlinear optical channel in a forward direction, the orthogonally
polarized optical bits and the second optical data bit each having
a forward polarization with respect to the nonlinear optical
channel; transmitting the orthogonally polarized optical bits and
the second optical data bit through the nonlinear optical channel
in a reverse direction, the orthogonally polarized optical bits and
the second optical data bit each having a reverse polarization
orientation with respect to the nonlinear optical channel, each of
the reverse polarization orientations being orthogonal to the
respective forward polarization orientation, wherein an optical
phase delay is imparted to the one of the orthogonally polarized
optical bits in response to the coincident transmission of the one
of the orthogonally polarized optical bits and the second optical
bit; and delaying one of the orthogonally polarized optical bits
relative to the other of the orthogonally polarized optical bits to
generate a temporal coincidence therebetween.
34. The method of claim 33 further comprising providing a result of
the logical operation in response to the optical phase delay
imparted to the one of the orthogonally polarized optical bits
relative to the other of the orthogonally polarized optical
bits.
35. The method of claim 33 wherein the optical phase delay imparted
to one of the first optical clock bit and the second optical clock
bit is an odd integer multiple of 180.degree..
36. The method of claim 33 wherein the logical operation is an AND
operation.
37. The method of claim 36 further comprising inverting one of the
first optical data bit and the second optical data bit prior to the
step of generating the pair of orthogonally polarized optical
bits.
38. A method for performing a logical operation on a first optical
data bit and a second optical data bit, the method comprising:
transmitting the first and second optical data bits, a first
optical clock bit and a second optical clock bit through a
nonlinear optical channel in a forward direction, the first and
second optical data bits and the first and second optical clock
bits each having a forward polarization with respect to the
nonlinear optical channel, the first and second optical clock bits
being orthogonally polarized and separated by a first time, the
first optical data bit and the second optical data bit being
substantially temporally coincident with the first optical clock
bit; transmitting the first and second optical data bits and the
first and second optical clock bits through the nonlinear optical
channel in a reverse direction, the first and second optical data
bits and the first and second optical clock bits each having a
reverse polarization orientation with respect to the nonlinear
optical channel, each of the reverse polarization orientations
being orthogonal to the respective forward polarization
orientation, wherein an optical phase delay is imparted to the
first optical clock bit if the first optical data bit is in an
asserted state, the second optical data bit is in an asserted state
or the first optical clock bit and the second optical clock bits
are in an asserted state; and delaying the first optical clock bit
relative to the second optical clock bit to generate a temporal
coincidence therebetween.
39. The method of claim 38 further comprising providing a result of
the logical operation in response to the optical phase delay
imparted to the first optical clock bit.
40. The method of claim 38 wherein the optical phase delay imparted
to the first optical clock bit is an odd integer multiple of
180.degree..
41. The method of claim 38 wherein the logical operation is an OR
operation.
42. The method of claim 38 wherein the logical operation is a NOR
operation.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to provisional U.S. patent
application serial No. 60/378,746, filed May 8, 2002, titled
"Polarization Stabilized All-Optical Switch," the entirety of which
provisional application is incorporated by reference herein.
FIELD OF THE INVENTION
[0003] The invention relates generally to optical switches. More
specifically, the invention relates to devices and methods for
regeneration and switching in high data rate optical communication
systems.
BACKGROUND
[0004] Typical optical networks are based on high-speed electronic
routers interconnected by optical fiber links utilizing wavelength
division multiplexing (WDM) transmission systems. To achieve packet
routing, the optical signal entering the router is converted to an
electrical signal (O/E conversion) and demultiplexed into
lower-rate data streams. The data streams are electronically routed
and multiplexed in a high-speed electrical signal that is generated
by the router for a specific optical wavelength.
[0005] As the demand for bandwidth in optical networks continues to
increase, the router must be able to perform its switching at
higher rates. The electronic signal bandwidth limitations incurred
during the O/E conversion and the conversion back to an optical
signal (E/O conversion), however, often result in router congestion
and reduced data throughput. In addition, the low energy optical
pulses received at the router after propagation over long distances
can be degraded, for example, by chromatic dispersion and nonlinear
effects in the transmission fiber. The optical pulses can also
exhibit timing variations (i.e., jitter) within a bit interval.
[0006] Conventional electronic signal regenerators provide pulse
amplification, pulse shaping and timing jitter correction of an
electrical signal. For WDM networks, an electronic signal
regenerator is required for each wavelength channel. After the O/E
conversion, the single wavelength channel electronic data is
demultiplexed into lower-rate data streams and each stream is
processed electronically. The lower-rate data streams are
multiplexed into a single data stream that is used to modulate the
desired wavelength optical carrier. The use of electronic
regenerators for re-amplification, re-shaping and re-timing in
high-speed optical networks is often prohibited by cost,
complexity, and power in many-channel WDM networks and sometimes is
limited by processing speed in ultrafast optical time domain
multiplexing (OTDM) networks.
[0007] All-optical switches eliminate the need for the O/E
conversion and E/O conversion. Consequently, all-optical switches
generally support higher data rates than electronic switches.
Optical switches utilizing nonlinearities in optical fibers are
subject to polarization instability due to temperature variations
and acoustic disturbances. Semiconductor materials are used as the
nonlinear media in some optical switches, however, the operational
speed of these switches is typically limited by multiple physical
processes. For example, long-lasting refractive index
nonlinearities cause performance degradations that are dependent on
the data statistics and the data rate.
SUMMARY OF THE INVENTION
[0008] In brief overview, the present invention relates to an
all-optical switch that is significantly insensitive to
polarization instabilities in the switch and, consequently, does
not require active polarization control. The optical switch can be
configured as an ultrafast logic gate, a switch for ultrafast
communication systems or a key component of an all-optical
regenerator. Performance is independent of the statistical
characteristics of the data controlling the switch.
[0009] In one aspect, the invention features an optical switch
having a birefringent optical channel, a coupler, a nonlinear
optical channel and a polarization rotation mirror. The
birefringent optical channel has a first port and a second port,
and provides a pair of orthogonally polarized optical pulses at the
second port in response to a first optical pulse received at the
first port. The orthogonally polarized optical pulses are separated
by a delay time. In one embodiment, the birefringent optical
channel is a polarization maintaining optical fiber. In another
embodiment, the birefringent optical channel is a birefringent
crystal. The coupler has a first port in optical communication with
the second port of the birefringent optical channel, a second port
adapted to receive a second optical pulse, and a third port. In one
embodiment, the optical switch includes a control optical channel
in optical communication with the second port of the coupler. The
nonlinear optical channel has a first port in optical communication
with the third port of the coupler. In one embodiment, the
nonlinear optical channel is a dispersion shifted optical fiber. In
another embodiment, the nonlinear optical channel is an optical
semiconductor. The polarization rotation mirror is in optical
communication with the second port of the nonlinear channel.
[0010] In another aspect, the invention features a method for
regenerating an optical data pulse. The method includes
transmitting the optical data pulse and a pair of orthogonally
polarized optical pulses through a nonlinear optical channel in a
forward direction. The optical data pulse and the orthogonally
polarized optical pulses each have a forward polarization
orientation with respect to the nonlinear optical channel. The
orthogonally polarized optical pulses are separated in time, and
the optical data pulse and one of the orthogonally polarized
optical pulses are substantially temporally coincident. The method
also includes transmitting the optical data pulse and the
orthogonally polarized optical pulses through the nonlinear optical
channel in a reverse direction wherein the optical data pulse and
the orthogonally polarized optical pulses each have a reverse
polarization orientation with respect to the nonlinear optical
channel. Each of the reverse polarization orientations is
orthogonal to a respective forward polarization orientation. An
optical phase delay is imparted to one of the orthogonally
polarized optical pulses relative to the other orthogonally
polarized optical pulse in response to the coincident transmission
of the optical data pulse and one of the orthogonally polarized
optical pulses through the nonlinear optical channel. The method
also includes the step of delaying one of the orthogonally
polarized optical pulses relative to the other orthogonally
polarized optical pulse to generate a temporal coincidence.
[0011] In another aspect, the invention features a method for
performing a logical operation of a first optical data bit and a
second optical data bit. The first and second optical data bits are
separated by a predetermined time. The method includes transmitting
the first and second optical data bits, a first optical clock bit
and a second optical clock bit through a nonlinear optical channel
in a forward direction. The first and second optical data bits and
the first and second optical clock bits each have a forward
polarization with respect to the nonlinear optical channel. In
addition, the first and second optical clock bits are orthogonally
polarized and separated in time. The first optical data bit is
substantially temporally coincident with the first optical clock
bit and the second optical data bit is substantially temporally
coincident with the second optical data bit. The method also
includes transmitting the optical data bits and the optical clock
bits through the nonlinear optical channel in a reverse direction.
The first and second optical data bits and the first and second
optical clock bits each have a reverse polarization orientation
with respect to the nonlinear optical channel. Each of the reverse
polarization orientations is orthogonal to a respective forward
polarization orientation. An optical phase delay is imparted to the
first optical clock bit if the first optical data bit is in an
asserted state and an optical phase delay is imparted to the second
optical clock bit if the second optical data bit is in an asserted
state. The method also includes delaying the first optical clock
bit relative to the second optical clock bit to generate a temporal
coincidence between the optical clock bits.
[0012] In another aspect, the invention features a method for
performing a logical operation on a first optical data bit and a
second optical data bit. The method includes generating a pair of
orthogonally polarized optical bits separated in time in response
to the first optical data bit. One of the orthogonally polarized
optical bits is substantially temporally coincident with the second
optical data bit. The method also includes transmitting the
orthogonally polarized optical bits and the second optical data bit
through a nonlinear optical channel in a forward direction wherein
the orthogonally polarized optical bits and the second optical data
bit each have a forward polarization with respect to the nonlinear
optical channel. The orthogonally polarized optical bits and the
second optical data bit are transmitted through the nonlinear
optical channel in a reverse direction wherein the orthogonally
polarized optical bits and the second optical data bit each have a
reverse polarization orientation with respect to the nonlinear
optical channel. Each of the reverse polarization orientations is
orthogonal to its respective forward polarization orientation. An
optical phase delay is imparted to the one of the orthogonally
polarized optical bits in response to the coincident transmission
of the one of the orthogonally polarized optical bits and the
second optical bit. One of the orthogonally polarized optical bits
is delayed relative to the other of the orthogonally polarized
optical bits to generate a temporal coincidence between the
orthogonally polarized bits.
[0013] In another embodiment of the method for performing a logical
operation on a first optical data bit and a second optical data
bit, the method includes transmitting the first and second optical
data bits, a first optical clock bit and a second optical clock bit
through a nonlinear optical channel in a forward direction wherein
the first and second optical data bits and the first and second
optical clock bits each have a forward polarization with respect to
the nonlinear optical channel. The first and second optical clock
bits are orthogonally polarized and separated by a first time. In
addition, the first optical data bit and the second optical data
bit are substantially temporally coincident with the first optical
clock bit. The method also includes transmitting the optical data
bits and the optical clock bits through the nonlinear optical
channel in a reverse direction wherein the first and second optical
data bits and the first and second optical clock bits each have a
reverse polarization orientation with respect to the nonlinear
optical channel. Each of the reverse polarization orientations is
orthogonal to a respective forward polarization orientation. An
optical phase delay is imparted to the first optical clock bit if
the first optical data bit is in an asserted state, the second
optical data bit is in an asserted state or the first optical clock
bit and the second optical clock bits are in an asserted state. The
method includes delaying the first optical clock bit relative to
the second optical clock bit to generate a temporal coincidence
between the optical clock bits.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The above and further advantages of this invention may be
better understood by referring to the following description in
conjunction with the accompanying drawings, in which like numerals
indicate like structural elements and features in various figures.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention.
[0015] FIG. 1 is a diagram of an embodiment of an optical switch in
accordance with the invention;
[0016] FIG. 2 is a plot of the normalized output intensity as a
function of delay between a data pulse and the local clock pulse
for the optical switch of FIG. 1;
[0017] FIG. 3 is a plot of the normalized dispersion in the
nonlinear medium as a function of wavelength for
multiple-wavelength data channels and multiple-wavelength local
clock channels for the optical switch of FIG. 1;
[0018] FIG. 4 is a diagram of another embodiment of an optical
switch including a local optical clock source and a clock recovery
circuit in accordance with the invention;
[0019] FIG. 5 is a flowchart representation of an embodiment of a
method for regenerating an optical data pulse in accordance with
the invention;
[0020] FIG. 6 is a flowchart representation of an embodiment of a
method for performing a logical operation on a first optical data
bit and a second optical data bit in accordance with the
invention;
[0021] FIG. 7 is a diagram of another embodiment of an optical
switch in accordance with the invention;
[0022] FIG. 8 is a flowchart representation of another embodiment
of a method for performing a logical operation on a first optical
data bit and a second optical data bit in accordance with the
invention;
[0023] FIG. 9 is a diagram of another embodiment of an optical
switch in accordance with the invention;
[0024] FIG. 10 is a flowchart representation of another embodiment
of a method for performing a logical operation on a first optical
data bit and a second optical data bit in accordance with the
invention; and
[0025] FIG. 11 is a diagram of another embodiment of an optical
switch in accordance with the invention.
DETAILED DESCRIPTION
[0026] FIG. 1 illustrates an all-optical switch 10 constructed in
accordance with the present invention. The switch 10 is configured
as a folded ultrafast nonlinear interferometer (FUNI). The switch
10 is a component of an all-optical 3R regenerator that
re-amplifies, re-shapes and re-times optical data pulses. Reshaping
removes distortion in an optical data pulse caused, for example, by
chromatic dispersion and nonlinear properties of the transmission
path. Re-timing removes any timing jitter (i.e., variation in pulse
position within a bit interval) present in an optical data pulse
stream. Effectively, the regenerator recovers the clock of the data
pulse stream and generates a stream of local clock pulses to match
the data pulse clock. The all-optical switch 10 outputs a local
transform-limited optical clock pulse if an optical data pulse is
present in the corresponding bit interval of the data pulse stream.
Conversely, no local optical clock pulse is output if the optical
data pulse is absent from the bit interval. In effect, the switch
10 operates logically as an AND gate with the local optical clock
pulses provided as a first input and the optical data pulses
provided as a second input.
[0027] The switch 10 includes a birefringent optical channel 14 and
a control optical channel 18 in optical communication with ports 22
and 26, respectively, of an optical coupler 30. In the illustrated
embodiment, the birefringent optical channel 14 is a birefringent
optical fiber (BRF) 14 such as a polarization maintaining optical
fiber (PMF). The birefringent axes of the BRF 14 are oriented to
the polarization of a local optical pulse 34 such that a
corresponding pair of orthogonally polarized (OP) optical pulses
38, 42 separated by a delay time t, exits the BRF 14. In another
embodiment (not shown), a highly birefringent crystal is used in
place of the BRF 14. The birefringent axes of the crystal are
similarly oriented with respect to the polarization of the local
optical pulse 34 to provide the OP pulses 38, 42. Use of the
birefringent crystal eliminates the need for the longer length of
BRF 14 and, therefore, results in a smaller switch package. The
control optical channel 18 is a single mode optical fiber coupled
to, or part of, an optical communications link. Optical data pulses
46 received at the switch input port 48 are transmitted from the
control optical channel 18 to the optical coupler 30. In one
embodiment the optical coupler 30 is a wavelength division
multiplexer (WDM) that achieves efficient coupling of different
wavelength local and network optical signals received at coupler
ports 22 and 26 into a combined optical signal at coupler port 50.
In another embodiment the optical coupler 30 is a beamsplitter-type
combiner (e.g., 3 dB combiner).
[0028] The switch 10 also includes a polarization rotation mirror
54 in optical communication with the coupler port 50 of the optical
coupler 30 through a nonlinear optical channel 58. In the
illustrated embodiment, the nonlinear optical channel 58 is a
dispersion shifted optical fiber (DSF). As described below, the
zero dispersion wavelength of the DSF 58 is selected to be
approximately the average value of the wavelength of the local
optical pulses 34 and the optical data pulses 46. In an alternative
embodiment (not shown) the nonlinear optical channel 58 is an
optical semiconductor (e.g., semiconductor optical amplifier
Alcatel model no. A1901SOA or OptoSpeed model no.
SOA1550MRI/X-250). The polarization rotation mirror 54 is a Faraday
mirror (e.g., E-Tek model no. HSFM20A0S4010) or other optical
component, or combination of components, that yields a reflected
optical pulse having a polarization that is orthogonal to the
polarization of the corresponding incident optical pulse. If the
optical coupler 30 is not wavelength discriminating (i.e. a WDM
coupler), the switch 10 includes a bandpass filter 62 (e.g., DiCon
model no. TF-1565-0.8-FC-3.0-1 or JDS model no. TB1570) disposed
between the BRF 14 and the data pulse port 22 of the optical
coupler 30. The switch also includes a polarization dependent
beamsplitter 66 (e.g., example JDS model no. PMCB2P0781000-001) in
optical communication with the BRF 14, and a three port optical
circulator 70 (e.g., JDS model no. CIRM23A231000-1) in optical
communication with the beamsplitter 66 and switch output port
78.
[0029] In operation, the switch 10 receives the stream of local
optical pulses 34 generated by a local optical clock source (not
shown). The local pulses 34 are routed through the circulator 70 to
the polarization dependent beamsplitter 66. Preferably the local
clock pulse 34 polarization is aligned for maximum transmission
through the beamsplitter 66 to the BRF 14. The birefringent axes of
the BRF 14 are aligned to the local optical pulses 34 so that the
differential propagation for orthogonally polarized components of
each local pulse 34 results in a pair of orthogonally polarized
(OP), equal-amplitude pulses 38, 42 exiting the birefringent
channel 14. For example, if the local pulses 34 are linearly
polarized, the birefringent axes of the BRF 14 are aligned at
approximately 45.degree. to the plane of polarization.
[0030] The exiting OP pulses 38, 42 are separated by a time delay t
according to the product of the birefringence magnitude and the
length of the BRF 14. Delay time t determines the temporal width of
the desired switching window for the switch 10. The minimum
allowable delay time t is determined by the width of the local
optical pulses 34. The switching window describes the ability of
the switch 10 to accommodate timing jitter in the data pulses 46 as
described below. OP pulses 38, 42 are transmitted through bandpass
filter 62 to port 22 of the optical coupler 30. Data pulses 46 are
transmitted over the control channel 18 to the second port 26 of
the optical coupler 30. Both the OP pulses 38, 42 and the data
pulses 46 are provided at the output port 50 of the optical coupler
30.
[0031] The polarization of the data pulses 46 is oriented to match
the polarization of one of the OP pulses 42. Because the data
pulses 46 are at a different wavelength than the OP pulses 38, 42,
the data pulses 46 propagate through the DSF 58 at a different
speed than the OP pulses 38, 42. If a data pulse 46 is present
during a bit interval, the data pulse 46 overlaps one of the OP
pulses 38, 42 for a period of time during forward transmission to
and reflected (i.e., reverse) transmission from the polarization
rotation mirror 54. As a result of nonlinear cross-phase modulation
during the period of temporal overlap, the commonly polarized OP
pulse 42 is shifted in optical phase relative to its complementary
OP pulse 38. Advantageously, any polarization instability due to
thermal and acoustic effects that occur during forward propagation
of the pulses 38, 42, 46 are reversed, or compensated, during
reverse propagation because of the orthogonal polarization rotation
induced by the polarization rotation mirror 54. This compensation
is achieved if the polarization instability does not vary over the
round trip propagation time through the DSF 58.
[0032] The spectral bandpass of the bandpass filter 62 is selected
so that the OP pulses 38, 42 are transmitted but the optical data
pulses 46 are rejected. Although all three optical pulses 38, 42,
46 are transmitted back through the optical coupler 30 to the
bandpass filter 62, only the OP pulses 38, 42 are transmitted to
the BRF 14. In other embodiments other types of spectral filters
are used in place of the bandpass filter 62. For example, a short
wavelength-pass spectral cutoff filter can be used if the
wavelength of the OP pulses 38, 42 is less than the wavelength of
the data pulses 46. Conversely, a long wavelength-pass spectral
filter can be used if the wavelength of the OP pulses 38, 42 is
greater than the wavelength of the data pulses.
[0033] The OP pulses 38, 42 continue in reverse propagation through
the BRF 14. Because the polarizations of the OP pulses 38, 42
during reverse propagation are orthogonal with respect to the
original polarization during forward propagation, the delay time t
induced during reverse propagation through the BRF 14 is opposite
that induced on forward transmission. In other words, the slower OP
pulse 38 during forward propagation through the BRF is the faster
pulse during reverse propagation through the BRF 14. Consequently,
returned OP pulses 38, 42 at the polarization dependent
beamsplitter 66 are temporally coincident. Effectively, a single
pulse substantially identical to the original local optical pulse
34 with an additional nonlinear phase shift and rotated
polarization propagates from the BRF 14 to the beamsplitter 66.
[0034] If the relative optical phase delay between the coincident
OP pulses 38, 42 is approximately zero, each pair of coincident OP
pulses 38, 42 combines at the polarization dependent beamsplitter
66 to generate a single complementary output pulse 82 having a
linear polarization oriented for reflection out of the beamsplitter
66. The complementary output pulse 82 is routed directly by the
beamsplitter 66 to the switch complementary output port 86.
Conversely, if the optical phase delay is such that the coincident
pulses 38, 42 are out of phase (i.e., 180.degree. relative phase
difference), the coincident OP pulses 38, 42 combine at the
polarization dependent beamsplitter 66 to generate a single output
pulse 74 having a linear polarization oriented for transmission
through the beamsplitter 66. The output pulse 74 is routed by the
circulator 70 to the switch output port 78.
[0035] The FUNI configured optical switch 10 uses a DSF as its
nonlinear medium rather than a BRF. If a BRF were used instead, its
high birefringence would cause a single data pulse 46 to pass
through multiple OP pulses 38, 42. This "walk-through" results in a
symmetric phase shift in the OP pulses 38, 42 that is dependent on
the number of data pulses 46 encountered. Consequently, the
switching contrast is significantly influenced by network data
statistics because a differential phase shift between OP pulses 38,
42 is required for switching. The use of low-birefringence DSF for
the nonlinear optical channel 58, however, limits single data pulse
46 walk-through to a single OP pulse 42. The wavelength separation
can be large as long as the difference in propagation speeds is
small. This goal is accomplished by establishing the two
wavelengths symmetrically (approximately) about the dispersion
zero. The difference in propagation speeds of the OP pulses 38, 42
and the data pulses 46 determines the degree of walk-through in the
nonlinear optical channel 58. A greater difference in propagation
speeds permits the switch 10 to operate over a larger timing
variation range between the data pulse 46 and its corresponding
local pulse 42. The performance of the switch 10 as a function of
time variation between data pulses 46 and local pulses 42 is
referred to as the "switching window". A large switching window is
required for a 3R regenerator to accommodate a large timing jitter
range.
[0036] FIG. 2 is a plot 116 of the normalized intensity of the
switch output pulses 74 as a function of temporal delay between the
data pulse 46 and the optical phase modulated OP pulses 38, 42 of
the FUNI configured optical switch 10 of FIG. 1. The horizontal
axis represents the delay between the data pulse 46 and one of the
OP pulses 38, 42 and the vertical axis represents the normalized
average intensity of the switch output pulse 74. Each peak 120, 124
corresponds to the coincidence of the data pulse 46 and one of the
corresponding OP pulses 38, 42. The temporal width t.sub.W of each
peak 120, 124 is determined by the differential group velocity
between the data pulse 46 and the OP pulses 38, 42 in the DSF 58.
The temporal separation t of the two peaks 120, 124 is determined
by the temporal separation between the OP pulses 38, 42.
[0037] In one embodiment, the optical switch 10 of FIG. 1 is
configured to operate in a WDM network in which a wavelength
multiplexed data pulse stream is provided at input port 48.
Referring also to FIG. 3, the normalized dispersion of the
nonlinear optical channel 58 is plotted for each data pulse stream,
designated D1, D2 and D3 (generally D), and its respective local
pulse stream, L1, L2 and L3 (generally L), respectively. Although
only three data pulse streams D and three local pulse streams L are
plotted for clarity, it should be recognized that the number of
data streams D and pulse streams L can be significantly greater.
The number of data streams D that can be accommodated by the switch
10 without significant performance degradation is ultimately
limited as described below.
[0038] The amount of pulse walk-through is determined according to
the differential dispersion of the nonlinear optical channel 58
between the wavelengths of the pulse streams D and L. The D and L
wavelength pairs are chosen to provide a small differential group
delay such that a small (e.g., few picoseconds) temporal
walk-through is achieved and the two pulse trains are tightly
coupled. However, the propagation speeds of data pulse D1 and local
pulse L1 are greater than the propagation speeds of data pulse D3
and local pulse L3. Consequently, data pulse D1 and local pulse L1
walk-through multiple data pulses D3 and local pulses L3. As the
difference in wavelength range between the first and third D and L
pulse streams increases, undesirable cross-phase modulation between
distinct D and L pulse streams can impart a sufficient relative
phase and the contrast of the output pulse stream 34 begins to
degrade.
[0039] FIG. 4 shows an embodiment of an all-optical switch 10'
constructed in accordance with the present invention. The switch
10' includes the components of the optical switch 10 of FIG. 1. In
addition, a clock recovery circuit 90 and an optical source 94
enable the switch 10' to maintain, or recover, the clock for the
optical data pulses 46. The clock recovery circuit 90 has an input
port 98 in optical communication with the output port 78 of the
optical switch 10'. The optical source 94 includes a control
terminal 102 in communication with an output terminal 106 of the
clock recovery circuit 90 and a source output port 110 in optical
communication with the optical circulator 70 through an optical
time delay module 112. In an alternative embodiment (not shown) the
input port 98 of the clock recovery circuit 90 is in optical
communication with the complementary output port 86 of the optical
switch 10'. In another alternative embodiment (not shown) the input
port 98 of the clock recovery circuit 90 is in optical
communication with the control input 48 in order to recover the
clock directly from the network data stream.
[0040] If the clock recovery circuit 90 is omitted from the switch
10', the local pulse stream 34 from the optical source 94 typically
does not remain synchronized to the data pulse stream 46. The clock
recovery circuit 90 serves to "lock" the pulse rate of the local
pulse stream 34 to the pulse rate of the data pulse stream 46. In
operation, the clock recovery circuit 90 provides a control signal
at its output terminal 106 that is responsive to a timing phase
shift between the local optical clock pulses 34 and the optical
data pulses 46. In one embodiment the clock recovery circuit 90
includes a phase lock loop (PLL) circuit (e.g., second-order
dithering PLL) and a RF driver circuit, and the optical source 94
includes a mode-locked fiber laser (e.g., mode-locked soliton
source). The PLL circuit modulates the RF source to generate a
control signal at the output terminal 106 of the clock recovery
circuit 90. In response, the pulse rate of the mode-locked laser is
changed to "track" the pulse rate of the optical data pulses
46.
[0041] The time delay module 112 delays the local optical pulses 34
in response to a delay signal generated by the clock recovery
circuit 90. This delay may be required if, for example, one or more
optical path lengths in the switch 10' change in time due to
temperature variations in the optical components which can cause an
undesirable timing shift between the local pulses and data pulses
34, 46. The time delay module 112 can be, for example, a
piezoelectric "stretcher" integrated with an optical fiber that
couples the local optical pulses 34 generated by the optical source
94 to the circulator 70. In an alternative configuration (not
shown) the time delay module 112 is in electrical communication
with the clock recovery circuit 90 and in optical communication
with port 26 of the WDM 30 and the switch input port 48. In this
alternative configuration the time delay module 112 delays the data
pulse 46 in response to the delay signal generated by the clock
recovery circuit 90.
[0042] FIG. 5 shows an embodiment of a method 150 for regenerating
an optical data pulse 46 using the optical switch 10 of FIG. 1. A
pair of OP pulses separated by a time t are generated (step 152) in
response to an optical clock pulse. The optical data pulse 46 and
the pair of OP optical pulses 38, 42 are transmitted (step 154)
through the nonlinear optical channel 58 in a forward direction.
The optical data pulse 46 is substantially temporally coincident
with one of the OP pulses 38, 42 such that the two optical pulses
(e.g. 42, 46) overlap for at least a portion of the forward
transmission and/or subsequent reverse transmission. In one
embodiment the wavelength of the data pulse 46 is different from
the wavelength of the OP pulses 38, 42. The three pulses 38, 42, 46
are transmitted (step 158) through the nonlinear optical channel 58
in a reverse direction after an orthogonal rotation of their
polarization components. Polarization rotation is achieved by the
polarization rotation mirror 54 (e.g., reflective Faraday rotator).
An optical phase delay is imparted to one of the OP pulses 42 as a
result of the walk-through with the data pulse 46 in the nonlinear
optical channel 58. In one embodiment the optical phase delay is an
odd integer multiple of 180.degree..
[0043] In step 162 one of the OP pulses 42 is delayed by a time t
relative to the other pulse 38 to "undo" or remove the original
delay t so that the pair of pulses 38, 42 are temporally
coincident. The delay t is imparted during reverse propagation
through the birefringent optical channel 14. A regenerated optical
pulse 74 is provided (step 166) at the switch output port 78 in
response to the imparted optical phase delay. In effect, the
regenerated output pulse 74 is the combination of the two OP
optical pulses 38, 42 into a single pulse having the proper
polarization orientation so that the polarization dependent
beamsplitter 66 routes the combined pulses 38, 42 to the output
port 78.
[0044] FIG. 6 shows an embodiment of a method 170 for performing a
logical operation on a first optical data bit and a second optical
data bit according to the optical switch of FIG. 1. It is to be
understood that the term "optical bit" as used herein refers to an
optical logic event and is generally interchangeable with the term
"optical pulse". Method 170 includes generating (step 174) a pair
of OP optical bits (pulses 38 and 42) in response to the first
optical data bit. The OP optical bits 38, 42 are separated by a
time t. One of the OP optical bits 42 is substantially temporally
coincident with the second optical data bit 46. The OP optical bits
38, 42 and the second optical data bit 46 are transmitted (step
178) through the nonlinear optical channel 58 in a forward
direction. The OP optical bits 38, 42 and the second optical bit 46
are then transmitted (step 182) through the nonlinear optical
channel 58 in a reverse direction. The polarizations of the optical
bits 38, 42, 46 during reverse transmission are orthogonal to the
respective polarization of the forward propagating optical bits due
to the polarization rotation mirror 54.
[0045] As previously described, the second optical data bit 46 and
OP optical bit 42 overlap for at least a portion of the forward
transmission and reverse transmission through the nonlinear optical
channel 58. An optical phase delay is imparted to the overlapped OP
optical bit 42 during the pulse walk-through. In one embodiment
this phase delay is an odd integer multiple of 180.degree.. After
reverse transmission is complete, one of the OP data bits 42 is
delayed (step 186) relative to the other OP data bit 38 to generate
a temporal coincidence. In step 190, a result of the logical
operation is provided at switch output port 78 in response to the
phase delay imparted to the overlapped OP optical bit 42 relative
to the other OP optical bit 38. In one embodiment the logical
operation is an AND operation. Thus, an output bit (pulse 74) is
present at the switch output port 78 only if both the first optical
bit 34 and the second optical bit 46 are present. In another
embodiment the logical operation is a Boolean AND operation between
the signal input 110 and the inverse of the control channel 48.
This operation is obtained by taking the complementary output 86 as
the switch output.
[0046] FIG. 7 depicts an all-optical switch 10" configured to
operate as an ultrafast Boolean exclusive-OR (XOR) logic gate
according to the present invention. All the components of the
all-optical switch 10 depicted in FIG. 1 are present in the same
configuration except that the bandpass filter 62' is disposed
between the circulator 70 and the output port 78 and a second
bandpass filter 62" is disposed between the beamsplitter 66 and the
complementary output port 86. The XOR gate 10" also includes a
second optical coupler 118 and an optical delay module 122. The
coupler 118 has a first coupler port 126 in optical communication
with a first gate input port 130 and a second coupler port 134 in
optical communication with the optical delay module 122. The
optical delay module 122 is also in communication with a second
gate input port 138.
[0047] In operation, the logic gate 10" performs the logical XOR
operation on logical optical input bit A and optical input bit B
(i.e., A XOR B) as summarized in Table 1. The optical delay module
122 delays optical bit B relative to optical bit A by a time t'
approximately equal to the delay time t between the OP pulses 38,
42. To achieve XOR functionality with logic gate 10", the optical
input bits A and B are either co-polarized with the OP pulses 38,
42 (i.e. same polarizations) or orthogonally aligned to the OP
pulse 38, 42 polarizations. The combination of optical bit stream A
and optical bit stream B is provided at the output port 142 of the
coupler 118. Combined optical bit stream A and B is combined by the
coupler 30 with the OP pulse stream 38, 42.
1 TABLE 1 Complementary Gate Input Gate Input Gate Output Gate Port
130 Port 138 Port 86 Output Port 78 A B A {overscore (XOR)} B A XOR
B 0 0 1 0 0 1 0 1 1 0 0 1 1 1 1 0
[0048] If both optical bit A and optical bit B are provided during
the same bit interval at the gate input ports 130 and 138,
respectively, optical bit A overlaps (or walks through) one OP
pulse 42 and optical bit B overlaps (or walks through) the other OP
pulse 38 during forward and reverse propagation through the
nonlinear optical channel 58. Thus, an optical phase shift is
induce by cross-phase modulation in each of the OP pulses 38, 42.
There is no change, however, to the relative optical phase between
the OP pulses 38, 42. Consequently, polarization rotation at the
polarization rotation mirror 54 and reverse propagation through the
birefringent optical channel 14 yields a single output pulse 82'
having a polarization orientation that is substantially orthogonal
to the orientation defined by the sum of the polarization vectors
of the OP pulses 38, 42. Consequently, the output pulse 82' is
reflected out of the polarization dependent beamsplitter 66 and
routed through the bandpass filter 62" to the complementary switch
output port 86 indicating {overscore (A XOR B)}=1.
[0049] If both data optical bits A and B are absent, there is no
optical phase shift imparted to either OP pulse 38, 42. Again,
there is no change to the relative optical phase between the
optical pulses 38, 42 and a single output pulse 82' is provided to
the switch complementary output port 86 indicating {overscore (A
XOR B)}=1.
[0050] If only one of the optical data bits A and B is present in
the bit interval, the corresponding overlapped OP pulse 38 or 42
acquires an optical phase shift during propagation through the
nonlinear optical channel 58. As a result, the relative optical
phase between the OP pulses 38, 42 is changed. The relative optical
phase change that is proportional to the product of the data bit A
or B intensity and the DSF 58 length and nonlinear index of
refraction is selected to be 180.degree.. After polarization
rotation occurs at the polarization rotation mirror 54, reverse
propagation of the OP pulses 38, 42 through the birefringent
optical channel 14 yields a single complementary output pulse 74'
having a polarization orientation that is substantially identical
to the polarization orientation defined by the sum of the
polarization vectors of the OP pulses 38, 42. Consequently, no
output pulse 82' is available at the switch output port 86.
Instead, the complementary output pulse 74' is directed by the
circulator 70 through bandpass filter 62' to the switch output port
78 indicating {overscore (A XOR B)}=1.
[0051] In contrast to the optical switch 10 of FIG. 1, the OP
optical pulses 38, 42 (combined as a logic output pulse 74' or 82',
respectively) pass through a bandpass filter 62' or 62" only once.
Accordingly, the optical loss imparted to the OP optical pulses 38,
42 for the XOR logic gate 10" are less. The two bandpass filters
62', 62" can be replaced by the single bandpass filter 62
positioned as depicted in FIG. 1 if the additional optical loss is
acceptable.
[0052] FIG. 8 shows an embodiment of a method 200 for performing a
logical operation on a first optical data bit and a second optical
data bit separated by a predetermined time t using the optical
switch of FIG. 7. Method 200 includes generating (step 202) a first
OP data bit A and a second OP data bit B separated by a time t' in
response to a pair of received optical data bits. A first OP clock
bit and a second OP clock bit (optical pulses 42 and 38,
respectively) separated by a time t are generated (step 204) in
response to an optical clock bit. The first and second OP data bits
A, B, and the first and second OP clock bits are transmitted (step
206) through the nonlinear optical channel 58 in a forward
direction. The first OP data bit A is substantially temporally
coincident with the first OP clock bit 42 such that the two bits A,
42 overlap during at least a portion of the forward transmission or
subsequent reverse transmission through the nonlinear optical
channel 58. Similarly, the second OP data bit B is substantially
temporally coincident with the second OP clock bit 38.
[0053] The method 200 also includes transmitting (step 208) the
first and second OP data bits A, B and the first and second OP
clock bits 42, 38 through the nonlinear optical channel 58 in a
reverse direction. Because of the presence of the polarization
rotation mirror 54, the polarizations of each of the OP data bits
A, B and the OP clock bits 38, 42 are orthogonal to their
respective polarizations during transmission in the forward
direction through the nonlinear optical channel 58. An optical
phase delay is imparted to the first OP clock bit 42 if the first
OP data bit A is in an asserted state (i.e., the optical pulse 46
is present for the corresponding bit interval). Similarly, an
optical phase delay is imparted to the second OP clock bit 38 if
the second OP data bit B is in an asserted state. In one embodiment
the phase delay imparted to either or both OP clock bits 38, 42 is
approximately an odd integer multiple of 180.degree.. The first OP
clock bit 42 is delayed (step 212) relative to the second OP clock
bit 38 so that the bits 38, 42 are temporally coincident.
[0054] The result of the logical operation is provided (step 216)
in response to the optical phase delays imparted to the first and
second OP clock bits 42, 38. In one embodiment the logical
operation is an XOR operation and the result is represented by
optical pulse 74' described above. In another embodiment the
logical operation is a complementary XOR operation and is
represented by optical pulse 82' described above.
[0055] FIG. 9 depicts an all-optical switch 10'" configured to
operate as a Boolean OR logic gate according to the present
invention. The components of the all-optical switch 10" depicted in
FIG. 7 are present in the same configuration except that the
optical delay module 122 is absent.
[0056] In operation, the all-optical switch 10'" performs the
logical OR operation on logical optical bit A and optical bit B
(i.e., A OR B) as summarized in Table 2.
2 TABLE 2 Complementary Gate Input Gate Input Gate Output Gate Port
130 Port 138 Port 86 Output Port 78 A B A NOR B A OR B 0 0 1 0 0 1
0 1 1 0 0 1 1 1 0 1
[0057] If both optical bit A and optical bit B are provided during
the same bit interval at the gate input ports 130 and 138,
respectively, the resulting higher amplitude optical pulse A+B
shown at the output port 142 of the logic coupler 118 is present.
However, the optical powers of the optical data bits A and B are
selected so that the switch 10'" operates in saturation. One
embodiment of this switch 10'" can be realized if the DSF nonlinear
optical channel 58 is replaced with a semiconductor optical
amplifier. In such an embodiment, the magnitude of the optical
phase shift imparted to the overlapped OP pulse either 38 or 42
during transmission through the nonlinear optical channel 58 is the
same regardless of whether one or both optical bits A and B are
present. Thus, the presence of at least one optical bit A, B during
the bit interval is sufficient to provide an output pulse 74' at
the switch output port 78 indicating A OR B=1 (or A NOR B=0). If
optical bit A and optical bit B are both absent during the bit
interval, an output pulse 82' is provided at the complementary
switch output port 86 indicating A OR B=0 (or A NOR B=1).
[0058] FIG. 10 shows an embodiment of a method 230 for performing a
logical operation on a first optical data bit and a second optical
data bit with reference to the optical switch 10'" of FIG. 9. The
method 230 includes generating (step 232) a first and a second OP
clock bits 42 and 38, respectively, in response to a received
optical clock pulse 34. The first and second optical data bits A
and B, respectively, and the first and second OP clock bits 42, 38
are transmitted (step 234) through the nonlinear optical channel 58
in a forward direction. The optical data bits A, B and the OP clock
bits 38, 42 are then transmitted (step 238) in a reverse direction
through the nonlinear optical channel 58 in a reverse direction.
The polarizations of each of the optical data bits A, B and the OP
clock bits 38, 42 are rotated by the polarization rotation mirror
54 such that they are orthogonal to their respective polarizations
during transmission in the forward direction through the nonlinear
optical channel 58. Because of the temporal coincidence of optical
bits A and B, and the first OP clock bit 42, an optical phase delay
is imparted to the first OP clock bit 42 if either optical data bit
A or B is in an asserted state. In one embodiment, the phase delay
imparted to either or both OP clock bits 38, 42 is approximately an
odd integer multiple of 180.degree.. The first OP clock bit 42 is
delayed (step 242) relative to the second OP clock bit 38 so that
the OP clock bits 38, 42 are temporally coincident. In step 246 the
result of the logical operation is provided at switch port 78 (or
switch port 86) in response to the optical phase delay imparted to
the first OP clock bit 42. In one embodiment the logical operation
is an OR operation and is represented by optical pulse 74' as
described above. In another embodiment the logical operation is a
NOR operation and is represented by optical pulse 82' as described
above.
[0059] Many other variations of the all-optical switch 10 of the
present invention are possible using the principles described
above. For example, FIG. 11 depicts an embodiment of an all-optical
switch 10.sup.iv configured to operate as a 2.times.2 optical
switch according to the present invention. The components of the
switch 10 depicted in FIG. 1 are present in the same configuration
except that an additional circulator 70' is included. The
additional circulator 70' is in optical communication with the
polarization dependent beamsplitter 66 and the complementary switch
output port 86. Data channels 1 and 2 provide optical pulses 34'
and 34", respectively, having orthogonal polarization orientations.
If control pulse 46' is present during a bit interval, the channel
1 optical pulse 34' is routed to the switch output port 78 and the
channel 2 optical pulse 34" is routed to the complementary switch
output port 86. Conversely, if control pulse 46' is absent during
the bit interval, the channel 2 optical pulse 34" is routed to the
switch output port 78 and the channel 1 optical pulse 34' is routed
to the complementary switch output port 86.
[0060] The optical switch of the present invention is not limited
to the embodiments described above. Those of ordinary skill in the
art will recognize that the optical switch can be configured to
perform other logical and processing functions. For example, many
data streams can be switched to any number of output ports
according to the principles described above. In addition, optical
switches can be cascaded or otherwise arranged to provide more
complex processing of optical data streams.
[0061] While the invention has been shown and described with
reference to specific preferred embodiments, it should be
understood by those skilled in the art that various changes in form
and detail may be made therein without departing from the spirit
and scope of the invention as defined by the following claims.
* * * * *